The spinal column consists of a complex system of bones and connective tissues that provides support for the body and protects the delicate spinal cord and nerves. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebral body includes an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces experienced by to the spinal column. A vertebral canal containing the spinal cord and nerves is located behind the vertebral bodies.
The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones cushioned by intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which includes the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bone (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine. Although complex, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Many types of spinal column disorders affect the function and integrity of the spine such as degenerative disc disease, spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine), scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine) and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain, as well as diminished nerve function.
Spinal pathologies limit the range, and/or threaten the critical elements of the nervous system housed within the spinal column. A variety of systems exist that achieve immobilization by implanting artificial assemblies in or on the spinal column. Lateral and anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Posterior implants generally include pairs of rods that are aligned along the axis which the bones are to be disposed, and which are then attached to the spinal column.
One of the most common motion segment surgical interventions is called arthrodesis, or spine fusion. Clinical success varies considerably, depending upon technique and indications, and consideration must be given to the concomitant risks and complications. For example, it has been shown that spine fusion decreases function by limiting the range of motion for patients in flexion, extension, rotation, and lateral bending. Furthermore, it has been shown that spine fusion creates increased stresses and, therefore, accelerated degeneration of adjacent non-fused motion segments. Also, the fusion device, whether artificial or biological, may migrate out of the fusion site.
While some disc replacement materials and methods currently are known, further improvements are needed. Thus, needs exist for placement of disc replacement materials and structures that may provide support and preserve motion.